Great Monday puzzle from Zamorca.
I thoroughly enjoyed this. Very neat clueing and witty surfaces. Difficult to pick favourites! Without a crosser to assist, 24d could be either GROWN or GROAN, depending on how you read it. My LOI was EXTREME with its tricky clueing. Thanks to Zamorca!

Across
1 Behave badly and smash model Roman hero skipping about (5,3,5)
BREAK THE RULES
BREAK (smash) + T (model) + HER[c]ULES (Roman hero, skipping C (about))
9 Electricity supplier for artillery unit (7)
BATTERY
Double definition
10 Spooner’s residence better for pet access (3-4)
CAT FLAP
A Spoonerism of: FLAT (residence) + CAP (better)
11 Description of first murder victim by lake (5)
LABEL
ABEL (first murder victim) by L (lake)
12 Communities plan a development including sea front area (9)
ESPLANADE
[communiti]ES PLAN A DE[development] (including)
13 Following initial destruction, gale gets mostly lower and dies down (8)
DWINDLES
D[estruction] (initial) + WIND (gale) + LES[s] (lower, mostly)
15 Grow old and conservative in New York office (6)
AGENCY
AGE (grow old) + C (conservative) in NY (New York)
18 Working with assistance close by (2,4)
ON HAND
ON (working) with HAND (assistance)
19 Cordial email sent off to secure taxi (8)
AMICABLE
(EMAIL)* (*sent off) to secure CAB
22 Go round by parking at hotel to reach bike track (5,4)
CYCLE PATH
CYCLE (go round) by P (parking) + AT + H (hotel)
24 Old lady’s getting minimum of tax allowance (5)
GRANT
GRAN (old lady) + T[ax] (minimum of)
25 Excite expert with very fine wine’s finish (7)
PROVOKE
PRO (expert) with V (very) + OK (fine) + [win]E (finish)
26 Best to run trip with church (7)
TROUNCE
(TO RUN)* (*trip) with CE (church)
27 Policeman’s severe runs treated with drug from community medic (8,5)
DISTRICT NURSE
DI (policeman) + STRICT (severe) + (RUNS)* (*treated) with E (drug)
Down
1 Little one briefly left in cot chattered (7)
BABBLED
BAB[y] (little one, briefly) + L (left) in BED (cot)
2 Found crack in prophet’s faith at last (9)
ESTABLISH
STAB (crack) in ELI’S (prophet’s) + [fait]H (at last)
3 Show reverence long after King’s born (5)
KNEEL
L (long) after K (king) + NEE (born)
4 Hospital always always admit fellow with allergy (3,5)
HAY FEVER
H (hospital) + AY (always); EVER (always) admits F (fellow)
5 Some epic errors turn up in cooking instructions (6)
RECIPE
EPIC ER[rors] (some, <turn up)
6 After nearly fatal resistance soldier gets cold and listless (9)
LETHARGIC
LETHA[l] (nearly fatal) + R (resistance) + GI (soldier) + C (cold)
7 Relish a sexy dance (5)
SALSA
Double definition
8 Notice surrounding river rising fast (6)
SPEEDY
SPY (notice) surrounding (DEE)< (river, <rising)
14 High-risk affair describing annoyance with America (9)
DANGEROUS
DO (affair) describing ANGER (annoyance) with US (America)
16 Baron, bafflingly popular with Queen, gives easy answer (2-7)
NO-BRAINER
(BARON)* (*bafflingly) + IN (popular) + ER (queen)
17 Certain quote brought up about politician has to be cut short (8)
EMPHATIC
CITE (quote, <brought up) about MP (politician)M + HA[s] (to be cut short)
18 Sit in gets hundreds supporting old university verging on penury (6)
OCCUPY
CC (hundreds) supporting O (old) + U (university) + P[enur]Y (verging on)
20 Old stories regularly have sweetheart married before end (7)
EXTREME
EX (old) + [s]T[o]R[i]E[s] (regularly) have [sw]E[et] (heart) + M (married, before)
21 One who minds about earth has a vocation (6)
CAREER
CARER (one who minds) about E (earth)
23 Husband in care of doctor’s given rise to multiple notes (5)
CHORD
H (husband) in CO (care of), DR (doctor, <given rise)
24 Escalated muttered complaint (5)
GROAN
“GROWN” (escalated, “muttered”)
*anagram
Couldnt decide so wrote GROIN
Thanks to Teacow and Zamorca. Lots of fun. I got GROAN but needed all the crossers to get EXTREME. I got CAT FLAP but not the CAP in the Spoonerism. A lot of ELIs recently and I enjoyed the HERcULES IN 1ac.
Thanks Teacow for parsing — I got 1a from “break” and the crossings but failed to fully understand where “the rules” came from. Failed to get GROAN (quite clever),GRANT, and EXTREME but the rest of the puzzle fell quickly. Liked NO-BRAINER a lot. Thanks Zamorca.
My 4th puzzle of the day, and just like the others it was very easy. Must be a Monday thing.
I couldn’t decide on GROWN or GROAN, so why wasn’t the clue changed before publication?
Was torn between GROWN and GROAN and luckily chose the correct answer. COD 1ac.
My fastest ever solve. Must have been straight forward.
Thanks to all.
Like Ong’ara, I was unsure about “grown” and “groan”. I guessed the former, but I see no reason to consider that “wrong”.
The solution in today’s paper is indeed given as GROAN. But agree it’s ambiguous.
Thanks Zamorca and Teacow
A quick solve finishing with one of the few clue types that get me a little – GROAN. This time I was able to pick where the invisible comma was … but these ones should always have the either / or letter as a crosser, surely !
Apart from that, it was quite an enjoyable solve with some lovely surfaces and a good variety of devices.
Finished in the SE corner with TROUNCE, EXTREME and that GROAN !!!
Yes, agreed that GROAN/GROAN was ambiguous. I also wasn’t happy with ESTABLISH – Eli wasn’t a prophet, he was a priest. Otherwise a pleasant enough diversion.